FIG. 1 is a view schematically illustrating the structure of a conventional liquid crystal display device of the type of double-sided display. In this liquid crystal display device 1, diffusion plates 13 and 14 are arranged on the front surface and on the back surface of a liquid crystal display panel 2, and a back light 3 comprising a source of light 9 and a light guide plate 10 is arranged on the back surface side of the liquid crystal display panel 2 through the diffusion plate 14 thereby to constitute a liquid crystal display portion 4 of the front surface side. Further, diffusion plates 15 and 16 are arranged on the front surface and on the back surface of a liquid crystal display panel 5, a back light 6 comprising a light source 11 and a liquid guide plate 12 is arranged on the back surface side of the liquid crystal display panel 5 through the diffusion plate 16 thereby to constitute a liquid crystal display portion 7 of the back surface side. The liquid crystal display portion 4 of the front surface side and the liquid crystal display portion 7 of the back surface side are overlapped back to back, and are partitioned by a double-sided reflector 8. The back lights 3 and 6 are turned on to illuminate the liquid crystal display panels 2 and 5 from the back surface sides, respectively.
In this liquid crystal display device, the two liquid crystal display portions comprising the liquid crystal display panel and the back light are arranged back to back resulting in an increase in the thickness of the liquid crystal display device arousing a problem in that equipment must have a large space for incorporating the liquid crystal display device. When the display is to be made on both surfaces, further, the two back lights must be turned on simultaneously, consuming electric power in increased amounts, which is not suited for portable equipment that uses rechargeable batteries. Besides, a back light and a liquid crystal display panel are necessary on the front surface side and on the back surface side, respectively, boosting up the cost.
Attempts have, therefore, been made to illuminate the liquid crystal display panels on both the front and back surfaces by using a single plane source of light. A liquid crystal display device of this kind has been disclosed in JP-A-2002-133906. FIG. 2 is a side view of the liquid crystal display device 21. FIG. 3 is a perspective view of a back light 22 used for the liquid crystal display device 21. The liquid crystal display device 21 of the type of emitting light from both surfaces includes liquid crystal display panels 23 and 24 arranged on both surfaces of the back light 22. In the back light 22, rod-like sources of light 26 such as cold cathode tubes are arranged at both edges of a light guide plate 25 in the form of a transparent flat plate, and leaving light control plates 28 are arranged on both surfaces of the light guide plate 25. A plurality of protruded portions 27 in the form of cylindrical lenses are formed on the surfaces of the leaving light control plates 28 facing the light guide plate 25, and the protruded portions 27 are intimately adhered at their central portions onto the surfaces of the light guide plate 25.
Light emitted from the rod-like sources of light 26 enters into the light guide plate 25 from the edges of the light guide plate 25 and propagates through the light guide plate 25. As shown in FIG. 4, light that has fallen on the contact surfaces between the protruded portions 27 and the light guide plate 25, falls on the leaving light control plates 28 from the light guide plate 25, totally reflected by the inner surfaces of the protruded portions 27 and goes out from the leaving light control plates 28 perpendicularly thereto. Therefore, upon arranging the liquid crystal display panel 23 and the liquid crystal display panel 24 on both sides of the back light 22, it is made possible to simultaneously illuminate the front and back liquid crystal display panels 23 and 24 by using one back light 22.
In this back light 22, however, light perpendicularly falling from the outer side as represented by a ray of light A in FIG. 4 transmits through the light guide plate 25 and the leaving light control plates 28, and goes out toward the front. When the transmission type liquid crystal display panels are used as liquid crystal display panels 23 and 24, therefore, light incident from the external side on one liquid crystal display panel, while the other liquid crystal display panel is being observed, transmits through the liquid crystal display panel and the back light, and is seen by the observer, arousing such problems that the image of the liquid crystal display panel of the back surface side is reflected on the image that is being observed and the color of the image is shaded.